Misha Dmitriev created HDFS-12051:
-------------------------------------
Summary: Intern INOdeFileAttributes$SnapshotCopy.name byte[]
arrays to save memory
Key: HDFS-12051
URL: https://issues.apache.org/jira/browse/HDFS-12051
Project: Hadoop HDFS
Issue Type: Improvement
Reporter: Misha Dmitriev
Assignee: Misha Dmitriev
When snapshot diff operation is performed in a NameNode that manages several
million HDFS files/directories, NN needs a lot of memory. Analyzing one heap
dump with jxray (www.jxray.com), we observed that duplicate byte[] arrays
result in 6.5% memory overhead, and most of these arrays are referenced by
{code}org.apache.hadoop.hdfs.server.namenode.INodeFileAttributes$SnapshotCopy.name{code}
and {code}org.apache.hadoop.hdfs.server.namenode.INodeFile.name{code}:
{code}
19. DUPLICATE PRIMITIVE ARRAYS
Types of duplicate objects:
Ovhd Num objs Num unique objs Class name
3,220,272K (6.5%) 104749528 25760871 byte[]
....
1,841,485K (3.7%), 53194037 dup arrays (13158094 unique)
3510556 of byte[17](112, 97, 114, 116, 45, 109, 45, 48, 48, 48, ...), 2228255
of byte[8](48, 48, 48, 48, 48, 48, 95, 48), 357439 of byte[17](112, 97, 114,
116, 45, 109, 45, 48, 48, 48, ...), 237395 of byte[8](48, 48, 48, 48, 48, 49,
95, 48), 227853 of byte[17](112, 97, 114, 116, 45, 109, 45, 48, 48, 48, ...),
179193 of byte[17](112, 97, 114, 116, 45, 109, 45, 48, 48, 48, ...), 169487 of
byte[8](48, 48, 48, 48, 48, 50, 95, 48), 145055 of byte[17](112, 97, 114, 116,
45, 109, 45, 48, 48, 48, ...), 128134 of byte[8](48, 48, 48, 48, 48, 51, 95,
48), 108265 of byte[17](112, 97, 114, 116, 45, 109, 45, 48, 48, 48, ...)
... and 45902395 more arrays, of which 13158084 are unique
<--
org.apache.hadoop.hdfs.server.namenode.INodeFileAttributes$SnapshotCopy.name
<-- org.apache.hadoop.hdfs.server.namenode.snapshot.FileDiff.snapshotINode <--
{j.u.ArrayList} <--
org.apache.hadoop.hdfs.server.namenode.snapshot.FileDiffList.diffs <--
org.apache.hadoop.hdfs.server.namenode.snapshot.FileWithSnapshotFeature.diffs
<-- org.apache.hadoop.hdfs.server.namenode.INode$Feature[] <--
org.apache.hadoop.hdfs.server.namenode.INodeFile.features <--
org.apache.hadoop.hdfs.server.blockmanagement.BlockInfo.bc <-- ... (1 elements)
... <-- org.apache.hadoop.hdfs.server.blockmanagement.BlocksMap$1.entries <--
org.apache.hadoop.hdfs.server.blockmanagement.BlocksMap.blocks <--
org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.blocksMap <--
org.apache.hadoop.hdfs.server.blockmanagement.BlockManager$BlockReportProcessingThread.this$0
<-- j.l.Thread[] <-- j.l.ThreadGroup.threads <-- j.l.Thread.group <-- Java
Static: org.apache.hadoop.fs.FileSystem$Statistics.STATS_DATA_CLEANER
409,830K (0.8%), 13482787 dup arrays (13260241 unique)
430 of byte[32](116, 97, 115, 107, 95, 49, 52, 57, 55, 48, ...), 353 of
byte[32](116, 97, 115, 107, 95, 49, 52, 57, 55, 48, ...), 352 of byte[32](116,
97, 115, 107, 95, 49, 52, 57, 55, 48, ...), 350 of byte[32](116, 97, 115, 107,
95, 49, 52, 57, 55, 48, ...), 342 of byte[32](116, 97, 115, 107, 95, 49, 52,
57, 55, 48, ...), 341 of byte[32](116, 97, 115, 107, 95, 49, 52, 57, 55, 48,
...), 341 of byte[32](116, 97, 115, 107, 95, 49, 52, 57, 55, 48, ...), 340 of
byte[32](116, 97, 115, 107, 95, 49, 52, 57, 55, 48, ...), 337 of byte[32](116,
97, 115, 107, 95, 49, 52, 57, 55, 48, ...), 334 of byte[32](116, 97, 115, 107,
95, 49, 52, 57, 55, 48, ...)
... and 13479257 more arrays, of which 13260231 are unique
<-- org.apache.hadoop.hdfs.server.namenode.INodeFile.name <--
org.apache.hadoop.hdfs.server.blockmanagement.BlockInfo.bc <--
org.apache.hadoop.util.LightWeightGSet$LinkedElement[] <--
org.apache.hadoop.hdfs.server.blockmanagement.BlocksMap$1.entries <--
org.apache.hadoop.hdfs.server.blockmanagement.BlocksMap.blocks <--
org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.blocksMap <--
org.apache.hadoop.hdfs.server.blockmanagement.BlockManager$BlockReportProcessingThread.this$0
<-- j.l.Thread[] <--
org.apache.hadoop.hdfs.server.blockmanagement.BlocksMap$1.entries <--
org.apache.hadoop.hdfs.server.blockmanagement.BlocksMap.blocks <--
org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.blocksMap <--
org.apache.hadoop.hdfs.server.blockmanagement.BlockManager$BlockReportProcessingThread.this$0
<-- j.l.Thread[] <-- j.l.ThreadGroup.threads <-- j.l.Thread.group <-- Java
Static: org.apache.hadoop.fs.FileSystem$Statistics.STATS_DATA_CLEANER
....
{code}
To eliminate this duplication and reclaim memory, we will need to write a small
class similar to StringInterner, but designed specifically for byte[] arrays.
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